1. Field of the Invention
This invention relates to instruments for locating the gravity vertical, and more specifically, to an instrument for projecting a dual pendulum stabilized light beam along a vertical line.
2. Description of the Prior Art
In construction the traditional tool for aligning a vertical structural member is the plumb bob. While inexpensive and simple to use, the plumb bob is limited by its sensitivity to structural vibration and air currents. At the other end of the economic spectrum is the jig transit, a very expensive instrument of great accuracy. Aside from its expense, the jib transit is delicate and more time consuming to operate, thus not well adapted for use by construction workers who must erect and align many vertical structural members in the course of a day.
It is generally known that a light beam or laser beam may be aligned with the gravity vertical to illuminate a reference point on an external surface vertically and spatially displaced from the light source. Typically, optical plumbing devices of this nature employ a light source mounted on a platform which may be aligned with the gravity vertical by means of one or more spirit levels and adjustable leveling legs. On a construction site this optical plumbing device has one great disadvantage. Rarely is the surface on which the device is placed found to be uniformly smooth and perfectly level. Thus the construction worker, moving the plumbing device from location to location, must constantly readjust the leveling legs to assure verticality. Furthermore, spirit levels are prone to parallax errors, and even minute parallax errors cause considerable deviations from the vertical when the beam is projected a great distance.
The present invention fills the economic and functional gap between the plumb bob and the jig transit by providing an extremely rugged and easy to use instrument for projecting a light beam along a vertical line. The invention has the further advantage of being extremely accurate regardless of the construction site terrain and requires no onsight adjustment by the construction worker.